Chromium plating



April 2, 1957 J. E. STARECK ETAL CHROMIUM PLATING Filed Aug. 12, 1954 l SUM OF 50 AND $iF5"'gms./Lifer GRAPH SHOWING VARIATION OF v Cr0 W'TH504 ANDSi/i;

0-0 gms./ Lifer coo INVENTDRS JESSE E. STARECK RONALD 00w I ATTORNEYS a s mf? I 2,586,558 .CHROMIMUMTP'LAITN" r teta- E. sandman wand Ronald-Dow, Detroit, Mich, assignprs,,- by mesnemssignments, to Metal & Thermit Corporation, New York, N. Y., a corporation 'of New Jersey Application August 12', r954; sirni P10142358 7 Clarita; 'ci. 204 51 'lfhis invention relates to ,chromiuin plating using. a chromicacid plating bathandparticularlyt to a method for producing crack-free deposits ofchromiuih. In 'co-pending application Serial No. 35 6,188, filed-.MaY 20, i953, now Patent 2,686,756, issued August .17, .1954, of which the presentapplication is acontinuation-in-part; there is described and claimed a method of producing crack-free chromium plate by maintaining, over a range of chromic acid concentration, particular and well defined amounts of catalyst acid radicals. whose concentrations are not only self-regulated butalso controlled by means of non-catalytic suppressor coinpounds and..by maintaining the temperature of the bath solution above a particular minimum value. .4

The present invention has tl e same purpose aslthe said copending t tpplic ation and utilizes similar, means for accomplishingthe purpose except that only aportion of. the catalyst acidrgdical content of, the bath solution is controlled by non-catalytic suppressor compounds. In. the present methodthebath solution comprises chromic acid, two catalyst-supplying compounds, namely, strontium sulfate and an alkali; metal silicofluoride, each having a limited solubility in the chromic acid bath, and a soluble non? catalytic alkali metal compound.v The alkali metal Ofthe non-catalytic compound is the same as that; of, the silicofiuoride compound and. is selected-fromthe. group. co'n': sist'ing of potassium and sodium. The non-catalyticcompound has the effect of controlling. the, concentratiomof. the dissolved silicofluoride radical in, theibathby .decreas; ing the solubilitvof the; alkali metal silicofluoride; V The concentrations of the dissolved sulfate and silicofluoride radicals are self-regulated, but -the sulfateconcentration is not controlled by a non-catalyticcompound. Specific non-catalytic compounds .are potassium or sodium..hydroxid e, potassium or sodium bichromate, potassium or sodium carbonate, and potassium or sodium chromate; ,Th concentration of the .chromic. acid, expressedas CrO3, in the bath is about 300 to 900 g /l, .The-arnount of the strontium sulfate and the alkali metal silicofluoride, is, in each case, suflicient to saturate the bath with dissolved sulfate and silicofluorideand to provide anin; soluble residueiof each ,saltin, the bath. The soluble noncatalytic alkali metallcom pound is pres llt in an amount sufficient to adjust. or suppress the concentration ofit the alkali v metal silicofiuoride in solution in the: bath from the .ui Sl ppres'sed' saturation, concentration of the latter to alower concentrationlsuchl.that.-thesurri f th di olved sulfate .(S04.=).,afndl.siIiCofiuoIidQ-lSiE v g./l, The concentrationof dissolved stilt is theunsup Y pressed saturation" concentration and maywa ry :fit 1 are and Sllicoll crO inth'e nia'n sf.,'l l b I msaawn he 2,787,588 a fieq.

acid concentration grams per liter with-the cop tion, ingrams per liter; of the sum ofdi s so lve d sulflatle and .silico'fiuoride. As-the chromic acid varies from 3 09 to 900 'g./l.,- the. required catalyst content variesjrom a low of 2.5 g./l. to a high of l0.5;g./l. Within the area ABCD, theplate that is deposited is .crac k-free and has theother advantages described hereinafter; At catalyst eoncentrationsthat.lieabovethe curve DCQ -the plate is cracked, and at catalystconcentrationsbelow-the curve AB, the.- plate .is nodular and-unsuitable. The abovedescribed variation of tIie-chroinic acid with the sum of the sulfate. and silicofluoridemay; also be shown" by Table. l,v .which also. shows -how the sulfate and silicofiuoride concentrations may individually vary:

...-.IABLE.- orot, sold-sure, SO ,g./I. SlF5,g./l. g./l. g./l.

300 2.5 I 1.5 1.0 400 2.5-3.9 1.5-2.2 1.0-2.4 500 a.0-5;2 1.5-2.8 1.0-3.7 600 3.5-6.5 1.5-3.5 1.0-5.0 700 4.0-7.8 1.5-4.2 1.0-6.3 800 4.5-0.2:- 1. 5-4.8 1.0-7.2 Uri- 7 J- -5.;l-Q- .-9...

Preferably, the amount of the. non-cata1ytic.. alkali metal compo 11d issuch ,asfto suppressgthe concentration of the silicofluoride in solution in the bath fromthe-unsuppressed saturation concentration of the latter to a lower concentration such that the sum of the dissolved sulfate (504:) :andsilicofluoride (SiFzjisizQ to 8, 5 gIAl. The. preferred concentration of dissolved sulfate mavvary from 1.8 to 5.0 g./l. and that of dissolve d sil icoflu ide from 1.2 to 7.l)'g./l., with the-preferred; sum of the two catalysts lyingsin the rangeof 3.9 to-8.5 g and he CrOs varyingIfrom 400. to;9 00-g:./l. This referred' variation of the sum of the dissolved catalys radicals with CrOa .concentrationis as defined by the area JKLM of the graph, and is also set forth in Table 2:

. A;more preferredrange of CrO concentration isjQQ to.,700 g./-l., with the dissolved sulfate varying'ifronrlfi to 3.-5- g. /l:, the .dissolvedsilicoflnoride varying'LfromZO toil) g./l.,',.and the sum of the two catalysts varying from 3.5.;10 6.5 gI/L: r f

Thev chromic acid: content =may be supplied by; adding chromit; acid -.as such.to the bath, although a portion mayv be supplied by the CrOs content of non-catalytic compounds like potassium on. sodium bichrornateand potassiuino r sodium chromate....lt will be-und-erstood that in referring to the .bathchromici-acid content-it is-intended to. include, .unless specified. otherwise, I the CrOa added-:per seia nd an yl.CrOg-addedinihe form of one of the foregoing non-catalyticcompoundsq-fi, I 3

The pla l s bath-m y. Ramad -rap in mixture of the solid ingredients. in POW e 'F S ra ul vriumJjs obtained. Th

bath" also be riiaiiitamed in this way. 7.

mersed in the bath. The bath is operated at a minimum temperature of 140 F., it having been found that below this temperature the plate that is produced is cracked. The upper temperature may extend to the boiling point of the bath but preferably is about 180 F. A preferred temperature range is 145 to 160 F. Generally, with higher temperatures the concentrations of dissolved ClOs, sulfate and silicofluoride should be higher, and higher current densities are used.

The crack-free quality of the chromium plate produced by the present method is not only determinable visually but also by means of other tests, as described in the said copending application. The crack-free plate of the invention is also characterized by having a hardness of about 425 to about 825, usually 550 to 700, Knoop. The plate is relatively soft and ductile as compared with conventional chromium plate. It has a smooth texture, a satiny finish, and a dull, matte, white color. It is easy to polish or buff.

The invention may be illustrated by the following examples:

Example 1 From a mixture of 410 parts of chromic acid, 284

parts of sodium dichromate, 14.3 parts of sodium silicofiuoride, and 7.2 parts of strontium sulfate, all parts by weight, an aqeous solution was made up analyzing as follows: 601 g./1. CrOs, 2.2 g./l. sulfate, and 1.8 g./1.

silicofluoride. A steel mandrel of conventional size was plated in this solution for 0.5 hour at 2.5 a. s. i. and 150 F. A dull, white crack-free deposit of chromium of 0.3 mil thickness was produced on the mandrel.

Example 3 A mixture of 446 parts of chromic acid, 80 parts of potassium dichromate, parts of potassium of silica fluoride, and 10 parts of strontium sulfate, all parts by weight, were dissolved in water to form a solution containing 500 g./ 1. of dissolved CrOs, 1.8 g./ 1. of sulfate, and 2.0 g./ 1. of silicotluoride. A steel mandrel of conventional size was plated in this solution at 150 F. and 3 a. s. i. for 1 hour, a smooth, dull white crack-free plate being produced.

In the examples and elsewhere the weight percentages of the ingredients of the solid mixture for making up the bath solution can also be expressed as parts by weight.

Deposits of varying thicknesses may be plated by the present method, ranging for example from a thickness just sufficient to cover the pores of the basis metal to any practical desired thicker deposit for which a demand may exist.

The invention is useful for articles made of any of a variety of basis metals, such as plain carbon steels, alloy steels including stainless steel, iron, cast iron, copper and copper alloys, nickel and nickel alloys, zinc and zinc alloys, aluminum and aluminum alloys, tin and tin alloys, lead and lead alloys, etc. In general, the basis metal may be any metal, or an undercoat on a metal, that can be chromium plated.

In the light of the foregoing description, the following is claimed:

I. A method of electrodepositing crack-free,.chromium plate on a metal article which comprises essentially: pass A steel rod 3" ing current in the range of 0.5 to 8 amperes per square inch from an anode to said article as a cathode immersed in an aqueous chromium plating bath at a temperature of F. to the boiling point of the bath, said bath comprising essentially 300 to 900 g./l. C1703, strontium sulfate and an alkali metal silicofluoride each in an amount sufficient to saturate said bath and to provide therein an undissolved residue of strontium sulfate and alkali metal silicofiuoride, respectively, and a soluble non-catalytic alkali metal compound in an amount sufiicient to suppress the concentration of the alkali metal silicofiuoride in solution in said bath so that the sum of the dissolved sulfate (804 and silicofiuoride (SiFs=) is in the range of 2.5 to 10.5 g./ 1., said alkali metal of the non-catalytic compound being the same as that of the alkali metal silicotluoride and being selected from the group consisting of sodium and potassium, said bath being free of compounds acting to suppress the concentration of dissolved sulfate, and said sum of dissolved sulfate and silicofiuoride varying with the CrOs concentration in the manner defined by the area ABCD of the graph shown in the accompanying drawing.

2. A method according to claim 1 in which the metal article that is chromium plated is an article of plain carbon steel.

3. A method according to claim 1 in which the temperature of the bath is in the range of 140 to 180 F.

4. A method according to claim 1 in which the alkali metal silicofluoride is potassium silicofluoride and the non-catalytic alkali metal compound is a potassium compound.

5. A method according to claim 1 in which the alkali metal silicofluoride is sodium silicofiuoride and the noncatalytic alkali metal compound is a sodium compound.

6. A method of electrodepositing crack-free, chromium plate on a metal article which comprises essentially: passing current in the range of 1 to 3 amperes per square inch from an anode to said article as a cathode immersed in an aqueous chromium plating bath at a temperature of to F., said bath comprising essentially 400 to 900 g./ 1. C103, strontium sulfate and an alkali metal silicofluoride each in an amount sufficient to saturate said bath and to provide therein an undissolved residue of strontium sulfate and alkali metal silicofluoride, respectively, and a soluble non-catalytic alkali metal compound in an amount sufficient to suppress the concentration of the alkali metal silicofluoride in solution in said bath so that the sum of the dissolved sulfate and silicofiuoride is in the range'of 3.0 to 8.5 g./l., said alkali metal of the noncatalytic compound being the same as that of the alkali metal silicofiuoride and being selected from the group consisting of sodium and potassium, said bath being free of compounds acting to suppress the concentration of dissolved sulfate, and said sum of dissolved sulfate and silicofluoride varying with the CrOs concentration in the manner defined by the area JKLM of the graph shown in the accompanying drawing.

7. An improved method of producing a cororsion re sistant chromium plated metal article comprising plating the unplated article, in a single plating operation, in an aqueous bath comprising essentially 300 to 900 g./l. CrOs, strontium sulfate and an alkali metal silicofiuoride each in an amount sufficient to saturate said bath and to provide therein an undissolved residue of strontium sulfate and alkali metal silicofiuoride, respectively, and a soluble non-catalytic alkali metal compound in an amount suflicient to suppress the concentration of the alkali metal silicofiuoride in solution in said bath from the unsuppressed saturation concentration of the latter compound to a lower concentration equivalent to 1.0 to 9.0 g./ 1. of dissolved silicofluoride (SiFs=), the concentration of dissolved sulfate (504 being from 1.5 to 5.5 g./ 1., the sum of said dissolved sulfate and silicofiuoride being in the range of 2.5 to 10.5 g./ 1., said bath being free of compounds acting to suppress the concentration of dissolved sulfate, said sum of dissolved sulfate and silicofluoride varying with the C103 concentration in the manner defined by the area ABCD of the graph shown in the accompanying drawing, the alkali metal of the non-catalytic compound and of the alkali metal silicofluoride being the same and being selected from the class consisting of potassium and sodium; performing the plating at a bath tem perature of 140 to 180 F. and a current density of 0.5

to 8 amperes per square inch; and thereby producing on UNIT ED STATES PATENTS Passal May 26, 1953 Stareck May 26, 1953 Stareck et a1. Aug. 17, 1954 

1. A METHOD OF ELECTRODEPOSITING CRACK-FREE CHROMIUM PLATE ON A METAL ARTICLE WHICH COMPRISES ESSENTIALLY: PASSING CURRENT IN THE RANGE OF 0.5 TO 8 AMPERES PER SQUARE INCH FROM AN ANODE TO SAID ARTICLE AS A CATHODE IMMERSED IN AN AQUEOUS CHROMIUM PLATING BATH AT A TEMPERATURE OF 140*F. TO THE BOILING POINT OF THE BATH, SAID BATH CONPRISING ESSENTIALLY 300 TO 900 G./1. CRO3, STRONTIUM SULFATE AND AN ALKALI METAL SILICONFLUORIDE EACH IN AN AMOUNT SUFFICIENT TO SATURATED SAID BATH AND TO PROVIDE THEREIN AN UNDISSOLVED RESIDUE OF STRONTIUM SULFATE AND ALKALI METAL SILICONFLUORIDE, RESPECTIVELY, AND A SOLUBLE NON-CATALYTIC ALKALI METAL COMPOUND IN AN AMOUNT SUFFICIENT TO SUPPRESS THE CONCENTRATION OF THE ALKALI METAL SILICOLFLUORIDE IN SOLUTION IN SAID BATH SO THAT THE SUM OF THE DISSOLVED SULFATE (SO4=) AND SILICOFLUORIDE (SIF6=) IS IN THE RANGE OF 2.5 TO 10.5 G./1., SAID ALKALI METAL OF THE NON-CATALYTIC COMPOUND BEING THE SAME AS THAT OF THE ALKALI METAL SILICOFLUORIDE AND BEING SELECTED FROM THE GROUP CONSISTING OF SODIUM AND POTASSIUM, SAID BATH BEING FREE OF COMPOUNDS ACTING TO SURPRESS THE CONCENTRATION OF DISSOLVED SULFATE, AND SAID SUM OF DISSOLVED SULFATE AND SILICONFLUORIDE VARYING WITH THE CRO3 CONCENTRATION IN THE MANNER DEFINED BY THE AREA ABCD OF THE GRAPH SHOWN IN THE ACCOMPANYING DRAWING. 